Experts respond: Maternal sleep habits and stillbirth risk

A New Zealand study published today in the British Medical Journal (BMJ) has examined whether snoring, sleep position, and other sleep practices in pregnant women are associated with risk of stillbirth.

Researchers at the University of Auckland found that sleeping on the left side may be associated with a reduced risk of stillbirth, when compared with sleeping on the right side or back. Snoring and other characteristics of sleep apnoea were not associated with an increased risk.

See a press release from the University of Auckland.

The study was based around interviews with 155 Auckland woman who had experienced a stillbirth at 28 weeks of pregnancy or later. Data from this sample was compared to a control group of 310 woman of similar health and background who had carried full term babies.

  • The absolute risk of late stillbirth for women who went to sleep on their left was 1.96/1000 and was 3.93/1000 for women who did not go to sleep on their left
  • Women who got up to go to the toilet once or less on the last night were more likely to experience a late stillbirth compared with women who got up more frequently (risk increased from 1.40/1000 to 3.71/1000)).
  • Women who regularly slept during the day in the previous month were also more likely to experience a late stillbirth than those who did not (increased from 1.26/1000 to 3.27/1000)).”

While the increased risk raises concerns, the study is preliminary and further research will be required to validate these results. As the authors state in the article:

“This is the first time that an association has been described between maternal sleep practices and late stillbirth risk, and the findings need to be treated with caution. Further studies, ideally with prospectively collected sleep data, are urgently needed to confirm or refute our findings.”

The Science Media Centre contacted experts for further comment on the research.

Dr Barry Taylor, Professor of Women’s and Children’s Health, University of Otago, who was involved in studies on cot death (SIDS) that led to changes in recommended sleeping position for infants, comments:

“This is a very interesting first step, and there are several plausible physiological mechanisms that could explain why the mother’s sleeping position might have an effect on the foetus. However, it is a long way yet from an intervention study, which you would ideally want before giving population-level advice, such as telling pregnant women to change their sleeping habits.

“The next step will be follow-up case control studies to see if the same results can be confirmed in multiple studies. In the case of cot death and sleeping studies with babies, that is what happened — all the case-control studies pointed the same direction. On the strength of that growing evidence and given the ethical concerns that would have been raised by a randomised-control trial at that stage, we went straight to public health advice [for putting infants to sleep on their backs].

“However, if you do skip over that final step of doing an intervention study, there are always lingering arguments over whether the intervention is directly responsible for the improvements that are seen. It is a difficult judgement call to make. In this case, because the risk of late stillbirth is so low overall, an intervention study would potentially require a very large group of mothers, perhaps more than 15,000, which is another factor to consider.”

Associate Professor Bob Hancox, Dunedin Multidiscplinary Health and Development Research Unit, University of Otago:

“This is an important and thought-provoking study. It suggests that sleeping on the left side during late pregnancy may be associated with a lower risk of stillbirth. It also suggests that getting up to go to the toilet frequently during the night and not taking daytime naps during late pregnancy may also reduce the risk of stillbirth.

“None of these observations have been made before and these are therefore new and potentially important findings. Both the researchers and commentators agree that they need to be confirmed in other studies before we will know whether these are real effects. I think that these are reasonable conclusions.

“The researchers did not have good measures of obstructive sleep apnoea, so we do not know for certain whether obstructive sleep apnoea could also contribute to stillbirth. However, snoring and daytime sleepiness, which are common features of obstructive sleep apnoea, were NOT associated with stillbirth.

“Why body position while asleep might matter is uncertain. The theory is that sleeping on the left side improves the blood supply to the uterus because the enlarged uterus does not constrict the blood supply as much as when lying on the right or back.

“What should pregnant women do about this – nothing at the moment. We just don’t know for certain yet, and the absolute risk of stillbirth is very small. We do need other researchers to confirm or disprove this association before we can make recommendations to pregnant women.”

An accompanying Editorial in the British Medical Journal from Dr Lucy Chappell (Kings College London) and Prof Gordon Smith (Cambridge University), titled “Should pregnant women sleep on their left?”, adds some context and perspective to the study’s findings.

“There is a strong possibility that part of the association can be explained by reverse causation. Reduced fetal movement is one of the most common symptoms seen before stillbirth. Moreover, in many cases delay occurs between intrauterine fetal death and its confirmation by a health professional. In a proportion of cases of stillbirth in this study, the baby may have died before the last sleep night reported by the mother.

“Compromised babies may have reduced movements in the days leading up to the death. Hence, rather than being a cause of stillbirth, the associations between longer sleep and not rising during the night in the week before stillbirth may reflect absent or reduced fetal movements, as a consequence of the baby’s death.

“A forceful campaign urging pregnant women to sleep on their left side is not yet warranted. Further research is needed before the link between maternal sleep position and risk of stillbirth can be regarded as strongly supported. If these findings are validated in a future study, advice on sleep position is an intervention that would be relatively easy to implement.

“The message is appealing, perhaps partly because of resonance with the campaign on changing sleep position for infants, which led to a marked reduction in rates of sudden infant death.However, the impact of a similar intervention in pregnancy is uncertain.

“Although the message for mothers to sleep on their left is probably harmless and may be helpful, this study should be seen as one that only generates a hypothesis that needs validation”

More information on stillbirth in a New Zealand context is available from the SMC website, collected following a special article series from the Lancet in April.
From the AusSMC

Associate Professor Vicki Flenady from the Australian and New Zealand Stillbirth Alliance:

“Prevention strategies to reduce the risk of stillbirth in late pregnancy remain limited.  Many of these deaths are unexplained, despite a thorough examination, leaving parents and care providers struggling with the reasons why. The static stillbirth rate for over 20 years, when all other mortality statistics have shown an improvement, suggests we need new leads.

“This interesting study by Stacey et al shows the requirement for large studies to identify new potential factors. This study is important because it generates new ideas for closer investigation in future studies. These future studies need to carefully take into account confounding factors which are linked to maternal sleep position, but where sleep position may not be the true reason for the death. Studies need to also address whether there is a biologically plausible mechanism that will allow us to understand how sleep position could result in stillbirth.

“The authors of the paper and the editorialist all agree that this is a preliminary result that needs to be confirmed. Based on the information that is available, expectant mothers should not change their behaviour. This is part of the continuing research effort to reduce the risk of stillbirth. Often research goes down blind alleys. We won’t know if this is a blind alley until further work is done.

“There are a number of known important risk factors for stillbirth for which we must offer advice and support including obesity, smoking and maternal age over 35 which contributes to around one-third of stillbirths. Smoking cessation programs in pregnancy are effective, however, many women are not provided with the support they need to stop smoking – this must be addressed as a priority. While we work to create awareness of these known risk factors, it is hoped that the planned study by ANZSA will help to resolve some of the unanswered questions on maternal sleep position and stillbirth.”


Jim Neilson, Professor of Obstetrics & Gynaecology, University of Liverpool and Liverpool Women’s Hospital said:

“The risk of having a stillborn baby in this study is broadly similar to the risk in the UK, as the New Zealand researchers did not include stillborn babies before 28 weeks, stillborn twins, or babies with malformations.

“There are important public health messages here – that women who are overweight or who smoke are more likely to have a stillborn baby.

“The link with sleeping patterns by the mother is much less clear. More research is needed. Until then, women need not change the position for sleeping in which they feel most comfortable.”

Ms Daghni Rajasingam, spokesperson for the Royal College of Obstetricians and Gynaecologists, said:

“There are many factors which are linked to stillbirth including obesity, increasing maternal age, ethnicity, congenital anomalies and placental conditions.  A significant number are unexplained.

“This small scale study looks at another possible factor, however, more research is needed into sleep patterns before any firm conclusions over sleeping positions can be made.  In the meantime, women should speak to their midwives if they are concerned.

“All new research into the causes of stillbirth is encouraging and is a step forward in understanding why they happen and improving stillbirth rates in the future.”

Dr Alexander Heazell, Walport Clinical Lecturer at the University of Manchester School of Medicine, said:

“Stillbirth affects in 1 in 200 births in the UK: over 4,000 per year. This study suggests that mothers sleeping on their left are less likely to have a stillbirth than those sleeping on their backs or on their right-hand side. However, even the risk of stillbirth reported in the study for mothers sleeping on their back or right-hand side (3.93 in 1,000) is lower than the current UK rate. It is too early to say whether we should encourage mothers to sleep on their left and more in depth studies are needed to confirm this study’s findings and to understand why sleeping on the left might reduce stillbirths.

“There are several weaknesses in the study, including the fact that mothers were asked to recall their sleeping position 25 days after experiencing a stillbirth. I agree with Professor Smith and Dr Chappell’s editorial, which describes these results as an interesting hypothesis that needs validation.”